1. Field of the Invention
The present invention relates to an electric motor and to a method of driving an electric motor.
Priority is claimed on Japanese Patent Application No. 2005-348981, filed Dec. 2, 2005, the contents of which are incorporated herein by reference.
2. Description of Related Art
Conventionally, an electric motor is known in which there are provided a first and second rotors that are provided concentrically around an axis of rotation of an electric motor, and the relative positions in the circumferential direction of the first and second rotors, namely, the phase difference thereof are controlled in accordance with the rotational speed of the electric motor, or in accordance with the speed of a rotating magnetic field that is generated in a stator (see, for example, Japanese Patent Application, First Publication No. 2002-204541).
In this electric motor, when the phase difference between the first rotor and the second rotor is controlled in accordance, for example, with the rotational speed of the electric motor, the relative positions in the circumferential direction of the first and second rotors are altered via components that become displaced in a radial direction by the action of centrifugal force. If the phase difference between the first and second rotors is controlled in accordance, for example, with the speed of the rotating magnetic field that is generated in the stator, then the relative positions in the circumferential direction of the first and second rotors are altered by altering the rotating magnetic field speed by supplying a control current to the stator coil while each rotor is maintaining its rotational speed by inertia.
In an electric motor according to an example of the above described conventional technology, for example, when the phase difference between the first and second rotors is controlled in accordance, for example, with the rotational speed of the electric motor, then it is only possible to control the phase difference between the first and second rotors when the electric motor is in operation, namely, when a centrifugal force corresponding to the rotational speed is in action, and the problem arises that it is not possible to control the phase difference at a suitable timing that includes when the electric motor is stopped. Moreover, in a state in which external vibration can easily affect the electric motor, such as when the electric motor is mounted as a driving source in a vehicle, then the further problem arises that it is difficult to appropriately control the phase difference between the first and second rotors solely by the action of centrifugal force. Furthermore, in this case, because the phase difference is controlled irrespective of variations in the power supply voltage of the power supply for the motor, there is a possibility, for example, that a failure will occur such as the size relationship between the power supply voltage and the counter-electromotive force of the electric motor becoming reversed.
In addition, if, for example, the phase difference between the first and second rotors is controlled in accordance with the speed of the rotating magnetic field that is generated in the stator, then because the speed of the rotating magnetic field is altered, the problem arises that processing to control the electric motor becomes extremely complex.